It can present itself in many forms, characterised by many different signs. The classical, chronic form is usually seen in 3-5 months old chickens, and is characterised by nerve lesions, resulting in paralysis.

A more acute form, develops in younger birds (more than 8 weeks), with tumours in intestines, livers and gonads.

The disease can manifest itself even earlier (3-4 weeks), and sometimes the only signs, showing that the chick is infected, are skin lesions.

Mortality due to Marek’s can be high, and losses may continue over a very long period. One never knows if and when it will stop. Prior to the use of vaccines, losses in affected flocks were estimated to range from a few birds to 25-30 % and occasionally as high as 60 %.

In addition to this, MD can be immunosuppressive, thus increasing the risk of other diseases. Chickens are usually infected with Marek's Disease virus in the first weeks of their life (first six weeks), but the signs of the disease usually develop late, sometimes not before the onset of lay. The later the infection, the less likely the birds develop signs of Marek’s. Usually they get infected early in life and carry the virus lifelong with them, shedding it and contaminating the surroundings. The virus is quite resistant and new chickens arriving at the premises will get infected, too.

Vaccines, when used properly and in combination with supportive measures such as hygiene, can be very effective.

Vaccination is the most effective means of disease control. Three classes of vaccine viruses are used to protect chickens against the disease: